As efforts are made to reduce motor vehicle exhaust emissions and to reduce air pollution, automobile manufacturers have turned toward the development of alternate fuel sources for motor vehicles. One of these fuel sources is compressed natural gas ("CNG"), an abundant, relatively inexpensive, and clean burning fuel. However, and unlike conventional hydrocarbon motor fuels, for example gasoline, compressed natural gas cannot be poured or dispensed as simply as hydrocarbon fuels may be, rather compressed natural gas is typically injected under pressure into a compressed natural gas vehicle cylinder.
As with gasoline powered vehicles, the on board storage capacity of the compressed natural gas vehicle cylinder, also referred to as the "cylinder", defines the maximum driving range of the motor vehicle before refueling is required. The underfilling of compressed natural gas vehicle cylinders, especially during fast fill charging operations, i.e., those taking less than five minutes, can occur at fueling stations having dispensers which are incorrectly or inaccurately compensated for initial cylinder and station supply gas pressures, as well as the supply gas temperature(s), the ambient temperature, and the dynamic fill conditions at the dispenser. At higher ambient temperature conditions, for example, those which exceed the "standard temperature" of 70 degrees fahrenheit, and under direct station compressor outlet charging of the cylinder, the underfilling of the cylinder can reach 20% or more of its rated gas mass storage capacity. This underfilling is a serious obstacle the natural gas industry must overcome in order to make compressed natural gas powered motor vehicles more acceptable by maximizing the driving range between cylinder fills. Moreover, this underfilling must be resolved without resorting to unnecessarily high fueling station gas storage pressures, or by extensively overpressurizing the cylinder during the fueling operation which may result in dangerous cylinder load conditions, and/or result in the venting of overpressurized compressed natural gas into the ambient air surrounding a motor vehicle, with the accompanying hazards of explosion or fire.
A primary cause of undercharging cylinders during fast fill operations is a result of fueling station dispensers which either ignore, or inaccurately estimate, the elevated compressed natural gas cylinder gas temperatures which occur in the charging process due to the compression, mixing, and other complex, transient, and dynamic thermodynamic processes, i.e., the conversion of gas enthalpy to temperature changes, resulting from the injection of compressed natural gas into a cylinder of generally unknown volume. This is shown graphically in FIG. 1, where the vehicle cylinder temperature is shown as a function of the change in injected gas mass for two cylinder volumes of 500 s.c.f. and 2,000 s.c.f., respectively, and at two initial cylinder gas pressures, 100 psi and 1500 psi, for each cylinder. As shown, if a cylinder is relatively full of gas when gas mass injection is started, shown by the 1,500 psi pressure lines, cylinder temperature rises in a generally linear manner which can be predicted to some degree. However, if initial cylinder pressure, and thus volume, is low cylinder temperatures change in a more unpredictable fashion making full cylinder fills difficult to determine. Another aspect of cylinder underfill problems is shown in FIG. 2, wherein three pairs of representative test data are shown, each pair starting at the same pressure and temperature in the cylinder. In FIG. 2 the temperatures shown in parentheses represents the average supply gas temperature over the fill process. Thus the importance of being able to accurately account for the supply conditions prior to and during the charging of the cylinder is shown by the differing endpoint gas temperatures and gas storage cylinder pressures resulting for each test pair due to only a difference in supply gas temperatures. This again demonstrates the dynamic nature of the compressed natural gas fill process. Yet another reason for the undercharging of cylinders is that the industry has not adopted a standard size cylinder for use in motor vehicles, and in some instances standard size cylinders cannot be used based due to the size of the motor vehicle as well as its intended load carrying capacity. This results in inaccuracies in the charging/fill process from the inability to accurately determine the volume of the cylinder, and thus the mass of compressed gas which can be injected into the cylinder to maximize the gas mass during the charging process.
As is known, during the charging or injection of compressed natural gas into a cylinder, the expansion of the gas in flowing from a station ground storage reservoir, or directly from a station compressor outlet, for example, tends generally to reduce the temperature of the compressed natural gas entering the cylinder due to the Joule-Thomson effect which occurs during this essentially constant enthalpy process, see FIG. 1. However, as the compressed natural gas enters the cylinder, the enthalpy of the gas is converted into internal energy, which equates to increases in internal cylinder gas temperature. The temperature range which results from this conversion of the compressed gas enthalpy into internal energy is a function not only of the size of the cylinder, however, but also of the pressure and temperature of the compressed gas being injected into the cylinder, as well as the pressure of the gas already in the cylinder prior to the injection of additional compressed natural gas, and the ambient temperature conditions at the dispenser. Thus, as the enthalpy of the compressed natural gas entering the cylinder is converted into internal energy within the cylinder, the gas undergoes complex and dynamic compression and mixing processes which typically overcome the cooling effect of the compressed natural gas being injected into the cylinder, resulting in increased cylinder temperatures which do not generally allow for to an accurate and complete injection of a "full" gas mass into the cylinder.
Most charging processes in the art are typically terminated when the fueling station dispenser measures, or estimates, the point of which the natural gas storage vehicle cylinder reaches a certain pressure level. Depending on the dispenser control system used, this level of cylinder cut-off pressure may have some dependence on ambient or station gas conditions, but typically fails to take into account the impact of the enthalpy to internal energy conversion which occurs during the fill process as it impacts cylinder pressures and temperatures. This will oftentimes result in an inaccurate or incomplete cylinder fill, which is especially problematic during fast fill charging operations. Although this problem may be lessened to some degree during a more protracted fill process, the expectations of consumers are that they will be able to fuel their motor vehicles quickly, efficiently, and safely in a fill process which will typically takes less than five minutes.
An example of a dispenser control system which employs a pre-calculated cutoff pressure scheme is the method and apparatus for dispensing compressed natural gas disclosed in U.S. Pat. No. 5,259,424 to Miller et al., issued Nov. 9, 1993. The control system of Miller et al calculates a vehicle tank cut-off pressure based on the ambient air temperature at the dispenser and the pre-programmed pressure rating of the vehicle cylinder stored in the control system. Miller et al. then calculate the volume of the vehicle tank and an additional mass of compressed natural gas required to increase the tank pressure to the cut-off pressure, whereupon the dispenser automatically turns off the compressed natural gas flow into the vehicle cylinder once the additional mass necessary to obtain the pre-calculated cut-off pressure has been injected into the cylinder. Although Miller at al. teach a method and apparatus which predetermines an amount of compressed natural gas, i.e., a gas mass, for injection into the gas cylinder, the mass of gas to be injected is based upon an estimated cut-off pressure within the vehicle cylinder, and is thus not a true mass based system which seeks to maximize the amount of gas mass injected into the cylinder, so long as the pressure limit of the cylinder is not exceeded.
Thus, and for the reasons discussed above, the temperatures that compressed natural gas vehicle cylinders reach at the end of dynamic fill or charging processes have been difficult to accurately predict in the known dispenser fill and control methodology. Thus, what is needed, but seemingly not available in the art, is a method and apparatus for dispensing compressed natural gas which compensates for the increase in cylinder gas temperatures during the charging process, and which also takes into account initial cylinder pressure and temperature conditions, as well as supply gas pressure and temperature conditions, in order to maximize the gas mass injected into a compressed natural gas vehicle cylinder for maximizing the driving range of a motor vehicle before the next fill process need be undertaken.